Coexisting differing erasure codes

ABSTRACT

A method for proactively rebuilding user data in a plurality of storage nodes of a storage cluster is provided. The method includes distributing user data and metadata throughout the plurality of storage nodes such that the plurality of storage nodes can read the user data, using erasure coding, despite loss of two of the storage nodes. The method includes determining that one of the storage nodes is unreachable and determining to rebuild the user data for the one of the storage nodes that is unreachable. The method includes reading the user data across a remainder of the plurality of storage nodes, using the erasure coding and writing the user data across the remainder of the plurality of storage nodes, using the erasure coding. A plurality of storage nodes within a single chassis that can proactively rebuild the user data stored within the storage nodes is also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a CON of Ser. No. 16/422,755 May 24, 2019 U.S. Pat.No. 11,036,583, Ser. No. 16/422,755 is a CON of Ser. No. 15/315,676 Dec.1, 2016 U.S. Pat. No. 10,303,547 Ser. No. 15/315,676 is a 371 of PCT/US15/34291 Jun. 4, 2015 PCT/US15/34291 is a CON of Ser. No. 14/296,160Jun. 4, 2014 U.S. Pat. No. 9,218,244.

BACKGROUND

Solid-state memory, such as flash, is currently in use in solid-statedrives (SSD) to augment or replace conventional hard disk drives (HDD),writable CD (compact disk) or writable DVD (digital versatile disk)drives, collectively known as spinning media, and tape drives, forstorage of large amounts of data. Flash and other solid-state memorieshave characteristics that differ from spinning media. Yet, manysolid-state drives are designed to conform to hard disk drive standardsfor compatibility reasons, which makes it difficult to provide enhancedfeatures or take advantage of unique aspects of flash and othersolid-state memory.

It is within this context that the embodiments arise.

SUMMARY

In some embodiments, a method for proactively rebuilding user data in aplurality of storage nodes of a storage cluster in a single chassis isprovided. The method includes distributing user data and metadatathroughout the plurality of storage nodes such that the plurality ofstorage nodes can read the user data, using erasure coding, despite lossof two of the plurality of storage nodes. The method includesdetermining that one of the plurality of storage nodes is unreachableand determining to rebuild the user data for the one of the plurality ofstorage nodes that is unreachable. The method includes reading the userdata across a remainder of the plurality of storage nodes, using theerasure coding and writing the user data, as recovered user data, acrossthe remainder of the plurality of storage nodes, using the erasurecoding. In some embodiments, reading the user data is performedaccording to a first erasure coding scheme and the writing is performedaccording to a differing second erasure coding scheme. A plurality ofstorage nodes within a single chassis that can proactively rebuild theuser data stored within the storage nodes is also provided.

Other aspects and advantages of the embodiments will become apparentfrom the following detailed description taken in conjunction with theaccompanying drawings which illustrate, by way of example, theprinciples of the described embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The described embodiments and the advantages thereof may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings. These drawings in no waylimit any changes in form and detail that may be made to the describedembodiments by one skilled in the art without departing from the spiritand scope of the described embodiments.

FIG. 1 is a perspective view of a storage cluster with multiple storagenodes and internal storage coupled to each storage node to providenetwork attached storage, in accordance with some embodiments.

FIG. 2 is a system diagram of an enterprise computing system, which canuse one or more of the storage clusters of FIG. 1 as a storage resourcein some embodiments.

FIG. 3 is a block diagram showing multiple storage nodes andnon-volatile solid state storage with differing capacities, suitable foruse in the storage cluster of FIG. 1 in accordance with someembodiments.

FIG. 4 is a block diagram showing an interconnect switch couplingmultiple storage nodes in accordance with some embodiments.

FIG. 5 is a multiple level block diagram, showing contents of a storagenode and contents of one of the non-volatile solid state storage unitsin accordance with some embodiments.

FIG. 6 is a flow diagram of a method for operating a storage cluster,which can be practiced on or by embodiments of the storage cluster,storage nodes and/or non-volatile solid state storages in accordancewith some embodiments.

FIG. 7 is an illustration showing an exemplary computing device whichmay implement the embodiments described herein.

DETAILED DESCRIPTION

The embodiments below describe a storage cluster that stores user data,such as user data originating from one or more user or client systems orother sources external to the storage cluster. The storage clusterdistributes user data across storage nodes housed within a chassis,using erasure coding and redundant copies of metadata. Erasure codingrefers to a method of data protection or reconstruction in which data isstored across a set of different locations, such as disks, storage nodesor geographic locations. Flash memory is one type of solid-state memorythat may be integrated with the embodiments, although the embodimentsmay be extended to other types of solid-state memory or other storagemedium, including non-solid state memory. Control of storage locationsand workloads are distributed across the storage locations in aclustered peer-to-peer system. Tasks such as mediating communicationsbetween the various storage nodes, detecting when a storage node hasbecome unavailable, and balancing I/Os (inputs and outputs) across thevarious storage nodes, are all handled on a distributed basis. Data islaid out or distributed across multiple storage nodes in data fragmentsor stripes that support data recovery in some embodiments. Ownership ofdata can be reassigned within a cluster, independent of input and outputpatterns. This architecture described in more detail below allows astorage node in the cluster to fail, with the system remainingoperational, since the data can be reconstructed from other storagenodes and thus remain available for input and output operations. Invarious embodiments, a storage node may be referred to as a clusternode, a blade, or a server.

The storage cluster is contained within a chassis, i.e., an enclosurehousing one or more storage nodes. A mechanism to provide power to eachstorage node, such as a power distribution bus, and a communicationmechanism, such as a communication bus that enables communicationbetween the storage nodes are included within the chassis. The storagecluster can run as an independent system in one location according tosome embodiments. In one embodiment, a chassis contains at least twoinstances of both the power distribution and the communication bus whichmay be enabled or disabled independently. The internal communication busmay be an Ethernet bus, however, other technologies such as PeripheralComponent Interconnect (PCI) Express, InfiniBand, and others, areequally suitable. The chassis provides a port for an externalcommunication bus for enabling communication between multiple chassis,directly or through a switch, and with client systems. The externalcommunication may use a technology such as Ethernet, InfiniBand, FibreChannel, etc. In some embodiments, the external communication bus usesdifferent communication bus technologies for inter-chassis and clientcommunication. If a switch is deployed within or between chassis, theswitch may act as a translation between multiple protocols ortechnologies. When multiple chassis are connected to define a storagecluster, the storage cluster may be accessed by a client using eitherproprietary interfaces or standard interfaces such as network filesystem (NFS), common internet file system (CIFS), small computer systeminterface (SCSI) or hypertext transfer protocol (HTTP). Translation fromthe client protocol may occur at the switch, chassis externalcommunication bus or within each storage node.

Each storage node may be one or more storage servers and each storageserver is connected to one or more non-volatile solid state memoryunits, which may be referred to as storage units. One embodimentincludes a single storage server in each storage node and between one toeight non-volatile solid state memory units, however this one example isnot meant to be limiting. The storage server may include a processor,dynamic random access memory (DRAM) and interfaces for the internalcommunication bus and power distribution for each of the power buses.Inside the storage node, the interfaces and storage unit share acommunication bus, e.g., PCI Express, in some embodiments. Thenon-volatile solid state memory units may directly access the internalcommunication bus interface through a storage node communication bus, orrequest the storage node to access the bus interface. The non-volatilesolid state memory unit contains an embedded central processing unit(CPU), solid state storage controller, and a quantity of solid statemass storage, e.g., between 2-32 terabytes (TB) in some embodiments. Anembedded volatile storage medium, such as DRAM, and an energy reserveapparatus are included in the non-volatile solid state memory unit. Insome embodiments, the energy reserve apparatus is a capacitor,super-capacitor, or battery that enables transferring a subset of DRAMcontents to a stable storage medium in the case of power loss. In someembodiments, the non-volatile solid state memory unit is constructedwith a storage class memory, such as phase change or magnetoresistiverandom access memory (MRAM) that substitutes for DRAM and enables areduced power hold-up apparatus.

One of many features of the storage nodes and non-volatile solid statestorage is the ability to proactively rebuild data in a storage cluster.The storage nodes and non-volatile solid state storage can determinewhen a storage node or non-volatile solid state storage in the storagecluster is unreachable, independent of whether there is an attempt toread data involving that storage node or non-volatile solid statestorage. The storage nodes and non-volatile solid state storage thencooperate to recover and rebuild the data in at least partially newlocations. This constitutes a proactive rebuild, in that the systemrebuilds data without waiting until the data is needed for a read accessinitiated from a client system employing the storage cluster. These andfurther details of the storage memory and operation thereof arediscussed below.

FIG. 1 is a perspective view of a storage cluster 160, with multiplestorage nodes 150 and internal solid-state memory coupled to eachstorage node to provide network attached storage or storage areanetwork, in accordance with some embodiments. A network attachedstorage, storage area network, or a storage cluster, or other storagememory, could include one or more storage clusters 160, each having oneor more storage nodes 150, in a flexible and reconfigurable arrangementof both the physical components and the amount of storage memoryprovided thereby. The storage cluster 160 is designed to fit in a rack,and one or more racks can be set up and populated as desired for thestorage memory. The storage cluster 160 has a chassis 138 havingmultiple slots 142. It should be appreciated that chassis 138 may bereferred to as a housing, enclosure, or rack unit. In one embodiment,the chassis 138 has fourteen slots 142, although other numbers of slotsare readily devised. For example, some embodiments have four slots,eight slots, sixteen slots, thirty-two slots, or other suitable numberof slots. Each slot 142 can accommodate one storage node 150 in someembodiments. Chassis 138 includes flaps 148 that can be utilized tomount the chassis 138 on a rack. Fans 144 provide air circulation forcooling of the storage nodes 150 and components thereof, although othercooling components could be used, or an embodiment could be devisedwithout cooling components. A switch fabric 146 couples storage nodes150 within chassis 138 together and to a network for communication tothe memory. In an embodiment depicted in FIG. 1 , the slots 142 to theleft of the switch fabric 146 and fans 144 are shown occupied by storagenodes 150, while the slots 142 to the right of the switch fabric 146 andfans 144 are empty and available for insertion of storage node 150 forillustrative purposes. This configuration is one example, and one ormore storage nodes 150 could occupy the slots 142 in various furtherarrangements. The storage node arrangements need not be sequential oradjacent in some embodiments. Storage nodes 150 are hot pluggable,meaning that a storage node 150 can be inserted into a slot 142 in thechassis 138, or removed from a slot 142, without stopping or poweringdown the system. Upon insertion or removal of storage node 150 from slot142, the system automatically reconfigures in order to recognize andadapt to the change. Reconfiguration, in some embodiments, includesrestoring redundancy and/or rebalancing data or load.

Each storage node 150 can have multiple components. In the embodimentshown here, the storage node 150 includes a printed circuit board 158populated by a CPU 156, i.e., processor, a memory 154 coupled to the CPU156, and a non-volatile solid state storage 152 coupled to the CPU 156,although other mountings and/or components could be used in furtherembodiments. The memory 154 has instructions which are executed by theCPU 156 and/or data operated on by the CPU 156. As further explainedbelow, the non-volatile solid state storage 152 includes flash or, infurther embodiments, other types of solid-state memory.

FIG. 2 is a system diagram of an enterprise computing system 102, whichcan use one or more of the storage nodes, storage clusters and/ornon-volatile solid state storage of FIG. 1 as a storage resource 108.For example, flash storage 128 of FIG. 2 may integrate the storagenodes, storage clusters and/or non-volatile solid state storage of FIG.1 in some embodiments. The enterprise computing system 102 hasprocessing resources 104, networking resources 106 and storage resources108, including flash storage 128. A flash controller 130 and flashmemory 132 are included in the flash storage 128. In variousembodiments, the flash storage 128 could include one or more storagenodes or storage clusters, with the flash controller 130 including theCPUs, and the flash memory 132 including the non-volatile solid statestorage of the storage nodes. In some embodiments flash memory 132 mayinclude different types of flash memory or the same type of flashmemory. The enterprise computing system 102 illustrates an environmentsuitable for deployment of the flash storage 128, although the flashstorage 128 could be used in other computing systems or devices, largeror smaller, or in variations of the enterprise computing system 102,with fewer or additional resources. The enterprise computing system 102can be coupled to a network 140, such as the Internet, in order toprovide or make use of services. For example, the enterprise computingsystem 102 could provide cloud services, physical computing resources,or virtual computing services.

In the enterprise computing system 102, various resources are arrangedand managed by various controllers. A processing controller 110 managesthe processing resources 104, which include processors 116 andrandom-access memory (RAM) 118. Networking controller 112 manages thenetworking resources 106, which include routers 120, switches 122, andservers 124. A storage controller 114 manages storage resources 108,which include hard drives 126 and flash storage 128. Other types ofprocessing resources, networking resources, and storage resources couldbe included with the embodiments. In some embodiments, the flash storage128 completely replaces the hard drives 126. The enterprise computingsystem 102 can provide or allocate the various resources as physicalcomputing resources, or in variations, as virtual computing resourcessupported by physical computing resources. For example, the variousresources could be implemented using one or more servers executingsoftware. Files or data objects, or other forms of data, are stored inthe storage resources 108.

In various embodiments, an enterprise computing system 102 could includemultiple racks populated by storage clusters, and these could be locatedin a single physical location such as in a cluster or a server farm. Inother embodiments the multiple racks could be located at multiplephysical locations such as in various cities, states or countries,connected by a network. Each of the racks, each of the storage clusters,each of the storage nodes, and each of the non-volatile solid statestorage could be individually configured with a respective amount ofstorage space, which is then reconfigurable independently of the others.Storage capacity can thus be flexibly added, upgraded, subtracted,recovered and/or reconfigured at each of the non-volatile solid statestorages. As mentioned previously, each storage node could implement oneor more servers in some embodiments.

FIG. 3 is a block diagram showing multiple storage nodes 150 andnon-volatile solid state storage 152 with differing capacities, suitablefor use in the chassis of FIG. 1 . Each storage node 150 can have one ormore units of non-volatile solid state storage 152. Each non-volatilesolid state storage 152 may include differing capacity from othernon-volatile solid state storage 152 on a storage node 150 or in otherstorage nodes 150 in some embodiments. Alternatively, all of thenon-volatile solid state storages 152 on a storage node or on multiplestorage nodes can have the same capacity or combinations of the sameand/or differing capacities. This flexibility is illustrated in FIG. 3 ,which shows an example of one storage node 150 having mixed non-volatilesolid state storage 152 of four, eight and thirty-two TB capacity,another storage node 150 having non-volatile solid state storage 152each of thirty-two TB capacity, and still another storage node havingnon-volatile solid state storage 152 each of eight TB capacity. Variousfurther combinations and capacities are readily devised in accordancewith the teachings herein. In the context of clustering, e.g.,clustering storage to form a storage cluster, a storage node can be orinclude a non-volatile solid state storage 152. Non-volatile solid statestorage 152 is a convenient clustering point as the non-volatile solidstate storage 152 may include a nonvolatile random access memory (NVRAM)component, as will be further described below.

Referring to FIGS. 1 and 3 , storage cluster 160 is scalable, meaningthat storage capacity with non-uniform storage sizes is readily added,as described above. One or more storage nodes 150 can be plugged into orremoved from each chassis and the storage cluster self-configures insome embodiments. Plug-in storage nodes 150, whether installed in achassis as delivered or later added, can have different sizes. Forexample, in one embodiment a storage node 150 can have any multiple of 4TB, e.g., 8 TB, 12 TB, 16 TB, 32 TB, etc. In further embodiments, astorage node 150 could have any multiple of other storage amounts orcapacities. Storage capacity of each storage node 150 is broadcast, andinfluences decisions of how to stripe the data. For maximum storageefficiency, an embodiment can self-configure as wide as possible in thestripe, subject to a predetermined requirement of continued operationwith loss of up to one, or up to two, non-volatile solid state storageunits 152 or storage nodes 150 within the chassis.

FIG. 4 is a block diagram showing a communications interconnect 170 andpower distribution bus 172 coupling multiple storage nodes 150.Referring back to FIG. 1 , the communications interconnect 170 can beincluded in or implemented with the switch fabric 146 in someembodiments. Where multiple storage clusters 160 occupy a rack, thecommunications interconnect 170 can be included in or implemented with atop of rack switch, in some embodiments. As illustrated in FIG. 4 ,storage cluster 160 is enclosed within a single chassis 138. Externalport 176 is coupled to storage nodes 150 through communicationsinterconnect 170, while external port 174 is coupled directly to astorage node. External power port 178 is coupled to power distributionbus 172. Storage nodes 150 may include varying amounts and differingcapacities of non-volatile solid state storage 152 as described withreference to FIG. 3 . In addition, one or more storage nodes 150 may bea compute only storage node as illustrated in FIG. 4 . Authorities 168are implemented on the non-volatile solid state storages 152, forexample as lists or other data structures stored in memory. In someembodiments the authorities are stored within the non-volatile solidstate storage 152 and supported by software executing on a controller orother processor of the non-volatile solid state storage 152. In afurther embodiment, authorities 168 are implemented on the storage nodes150, for example as lists or other data structures stored in the memory154 and supported by software executing on the CPU 156 of the storagenode 150. Authorities 168 control how and where data is stored in thenon-volatile solid state storages 152 in some embodiments. This controlassists in determining which type of erasure coding scheme is applied tothe data, and which storage nodes 150 have which portions of the data.Each authority 168 may be assigned to a non-volatile solid state storage152. Each authority may control a range of inode numbers, segmentnumbers, or other data identifiers which are assigned to data by a filesystem, by the storage nodes 150, or by the non-volatile solid statestorage 152, in various embodiments.

Every piece of data, and every piece of metadata, has redundancy in thesystem in some embodiments. In addition, every piece of data and everypiece of metadata has an owner, which may be referred to as anauthority. If that authority is unreachable, for example through failureof a storage node, there is a plan of succession for how to find thatdata or that metadata. In various embodiments, there are redundantcopies of authorities 168. Authorities 168 have a relationship tostorage nodes 150 and non-volatile solid state storage 152 in someembodiments. Each authority 168, covering a range of data segmentnumbers or other identifiers of the data, may be assigned to a specificnon-volatile solid state storage 152. In some embodiments theauthorities 168 for all of such ranges are distributed over thenon-volatile solid state storages 152 of a storage cluster. Each storagenode 150 has a network port that provides access to the non-volatilesolid state storage(s) 152 of that storage node 150. Data can be storedin a segment, which is associated with a segment number and that segmentnumber is an indirection for a configuration of a RAID (redundant arrayof independent disks) stripe in some embodiments. The assignment and useof the authorities 168 thus establishes an indirection to data.Indirection may be referred to as the ability to reference dataindirectly, in this case via an authority 168, in accordance with someembodiments. A segment identifies a set of non-volatile solid statestorage 152 and a local identifier into the set of non-volatile solidstate storage 152 that may contain data. In some embodiments, the localidentifier is an offset into the device and may be reused sequentiallyby multiple segments. In other embodiments the local identifier isunique for a specific segment and never reused. The offsets in thenon-volatile solid state storage 152 are applied to locating data forwriting to or reading from the non-volatile solid state storage 152 (inthe form of a RAID stripe). Data is striped across multiple units ofnon-volatile solid state storage 152, which may include or be differentfrom the non-volatile solid state storage 152 having the authority 168for a particular data segment.

If there is a change in where a particular segment of data is located,e.g., during a data move or a data reconstruction, the authority 168 forthat data segment should be consulted, at that non-volatile solid statestorage 152 or storage node 150 having that authority 168. In order tolocate a particular piece of data, embodiments calculate a hash valuefor a data segment or apply an inode number or a data segment number.The output of this operation points to a non-volatile solid statestorage 152 having the authority 168 for that particular piece of data.In some embodiments there are two stages to this operation. The firststage maps an entity identifier (ID), e.g., a segment number, inodenumber, or directory number to an authority identifier. This mapping mayinclude a calculation such as a hash or a bit mask. The second stage ismapping the authority identifier to a particular non-volatile solidstate storage 152, which may be done through an explicit mapping. Theoperation is repeatable, so that when the calculation is performed, theresult of the calculation repeatably and reliably points to a particularnon-volatile solid state storage 152 having that authority 168. Theoperation may include the set of reachable storage nodes as input. Ifthe set of reachable non-volatile solid state storage units changes theoptimal set changes. In some embodiments, the persisted value is thecurrent assignment (which is always true) and the calculated value isthe target assignment the cluster will attempt to reconfigure towards.This calculation may be used to determine the optimal non-volatile solidstate storage 152 for an authority in the presence of a set ofnon-volatile solid state storage 152 that are reachable and constitutethe same cluster. The calculation also determines an ordered set of peernon-volatile solid state storage 152 that will also record the authorityto non-volatile solid state storage mapping so that the authority may bedetermined even if the assigned non-volatile solid state storage isunreachable. A duplicate or substitute authority 168 may be consulted ifa specific authority 168 is unavailable in some embodiments.

With reference to FIGS. 1-4 , two of the many tasks of the CPU 156 on astorage node 150 are to break up write data, and reassemble read data.When the system has determined that data is to be written, the authority168 for that data is located as above. When the segment ID for data isalready determined the request to write is forwarded to the non-volatilesolid state storage 152 currently determined to be the host of theauthority 168 determined from the segment. The host CPU 156 of thestorage node 150, on which the non-volatile solid state storage 152 andcorresponding authority 168 reside, then breaks up or shards the dataand transmits the data out to various non-volatile solid state storage152. The transmitted data is written as a data stripe in accordance withan erasure coding scheme. In some embodiments, data is requested to bepulled, and in other embodiments, data is pushed. In reverse, when datais read, the authority 168 for the segment ID containing the data islocated as described above. The host CPU 156 of the storage node 150 onwhich the non-volatile solid state storage 152 and correspondingauthority 168 reside requests the data from the non-volatile solid statestorage and corresponding storage nodes pointed to by the authority. Insome embodiments the data is read from flash storage as a data stripe.The host CPU 156 of storage node 150 then reassembles the read data,correcting any errors (if present) according to the appropriate erasurecoding scheme, and forwards the reassembled data to the network. Infurther embodiments, some or all of these tasks can be handled in thenon-volatile solid state storage 152. In some embodiments, the segmenthost requests the data be sent to storage node 150 by requesting pagesfrom storage and then sending the data to the storage node making theoriginal request.

In some systems, for example in UNIX-style file systems, data is handledwith an index node or inode, which specifies a data structure thatrepresents an object in a file system. The object could be a file or adirectory, for example. Metadata may accompany the object, as attributessuch as permission data and a creation timestamp, among otherattributes. A segment number could be assigned to all or a portion ofsuch an object in a file system. In other systems, data segments arehandled with a segment number assigned elsewhere. For purposes ofdiscussion, the unit of distribution is an entity, and an entity can bea file, a directory or a segment. That is, entities are units of data ormetadata stored by a storage system. Entities are grouped into setscalled authorities. Each authority has an authority owner, which is astorage node that has the exclusive right to update the entities in theauthority. In other words, a storage node contains the authority, andthat the authority, in turn, contains entities.

A segment is a logical container of data in accordance with someembodiments. A segment is an address space between medium address spaceand physical flash locations, i.e., the data segment number, are in thisaddress space. Segments may also contain metadata, which enable dataredundancy to be restored (rewritten to different flash locations ordevices) without the involvement of higher level software. In oneembodiment, an internal format of a segment contains client data andmedium mappings to determine the position of that data. Each datasegment is protected, e.g., from memory and other failures, by breakingthe segment into a number of data and parity shards, where applicable.The data and parity shards are distributed, i.e., striped, acrossnon-volatile solid state storage 152 coupled to the host CPUs 156 (SeeFIG. 5 ) in accordance with an erasure coding scheme. Usage of the termsegments refers to the container and its place in the address space ofsegments in some embodiments. Usage of the term stripe refers to thesame set of shards as a segment and includes how the shards aredistributed along with redundancy or parity information in accordancewith some embodiments.

A series of address-space transformations takes place across an entirestorage system. At the top is the directory entries (file names) whichlink to an inode. Modes point into medium address space, where data islogically stored. Medium addresses may be mapped through a series ofindirect mediums to spread the load of large files, or implement dataservices like deduplication or snapshots. Medium addresses may be mappedthrough a series of indirect mediums to spread the load of large files,or implement data services like deduplication or snapshots. Segmentaddresses are then translated into physical flash locations. Physicalflash locations have an address range bounded by the amount of flash inthe system in accordance with some embodiments. Medium addresses andsegment addresses are logical containers, and in some embodiments use a128 bit or larger identifier so as to be practically infinite, with alikelihood of reuse calculated as longer than the expected life of thesystem. Addresses from logical containers are allocated in ahierarchical fashion in some embodiments. Initially, each non-volatilesolid state storage 152 may be assigned a range of address space. Withinthis assigned range, the non-volatile solid state storage 152 is able toallocate addresses without synchronization with other non-volatile solidstate storage 152.

Data and metadata is stored by a set of underlying storage layouts thatare optimized for varying workload patterns and storage devices. Theselayouts incorporate multiple redundancy schemes, compression formats andindex algorithms. Some of these layouts store information aboutauthorities and authority masters, while others store file metadata andfile data. The redundancy schemes include error correction codes thattolerate corrupted bits within a single storage device (such as a NANDflash chip), erasure codes that tolerate the failure of multiple storagenodes, and replication schemes that tolerate data center or regionalfailures. In some embodiments, low density parity check (LDPC) code isused within a single storage unit. Reed-Solomon encoding is used withina storage cluster, and mirroring is used within a storage grid in someembodiments. Metadata may be stored using an ordered log structuredindex (such as a Log Structured Merge Tree), and large data may not bestored in a log structured layout.

In order to maintain consistency across multiple copies of an entity,the storage nodes agree implicitly on two things through calculations:(1) the authority that contains the entity, and (2) the storage nodethat contains the authority. The assignment of entities to authoritiescan be done by pseudorandomly assigning entities to authorities, bysplitting entities into ranges based upon an externally produced key, orby placing a single entity into each authority. Examples of pseudorandomschemes are linear hashing and the Replication Under Scalable Hashing(RUSH) family of hashes, including Controlled Replication Under ScalableHashing (CRUSH). In some embodiments, pseudo-random assignment isutilized only for assigning authorities to nodes because the set ofnodes can change. The set of authorities cannot change so any subjectivefunction may be applied in these embodiments. Some placement schemesautomatically place authorities on storage nodes, while other placementschemes rely on an explicit mapping of authorities to storage nodes. Insome embodiments, a pseudorandom scheme is utilized to map from eachauthority to a set of candidate authority owners. A pseudorandom datadistribution function related to CRUSH may assign authorities to storagenodes and create a list of where the authorities are assigned. Eachstorage node has a copy of the pseudorandom data distribution function,and can arrive at the same calculation for distributing, and laterfinding or locating an authority. Each of the pseudorandom schemesrequires the reachable set of storage nodes as input in some embodimentsin order to conclude the same target nodes. Once an entity has beenplaced in an authority, the entity may be stored on physical devices sothat no expected failure will lead to unexpected data loss. In someembodiments, rebalancing algorithms attempt to store the copies of allentities within an authority in the same layout and on the same set ofmachines.

Examples of expected failures include device failures, stolen machines,datacenter fires, and regional disasters, such as nuclear or geologicalevents. Different failures lead to different levels of acceptable dataloss. In some embodiments, a stolen storage node impacts neither thesecurity nor the reliability of the system, while depending on systemconfiguration, a regional event could lead to no loss of data, a fewseconds or minutes of lost updates, or even complete data loss.

In the embodiments, the placement of data for storage redundancy isindependent of the placement of authorities for data consistency. Insome embodiments, storage nodes that contain authorities do not containany persistent storage. Instead, the storage nodes are connected tonon-volatile solid state storage units that do not contain authorities.The communications interconnect between storage nodes and non-volatilesolid state storage units consists of multiple communicationtechnologies and has non-uniform performance and fault tolerancecharacteristics. In some embodiments, as mentioned above, non-volatilesolid state storage units are connected to storage nodes via PCIexpress, storage nodes are connected together within a single chassisusing an Ethernet backplane, and chassis are connected together to forma storage cluster. Storage clusters are connected to clients usingEthernet or fiber channel in some embodiments. If multiple storageclusters are configured into a storage grid, the multiple storageclusters are connected using the Internet or other long-distancenetworking links, such as a “metro scale” link or private link that doesnot traverse the internet.

Authority owners have the exclusive right to modify entities, to migrateentities from one non-volatile solid state storage unit to anothernon-volatile solid state storage unit, and to add and remove copies ofentities. This allows for maintaining the redundancy of the underlyingdata. When an authority owner fails, is going to be decommissioned, oris overloaded, the authority is transferred to a new storage node.Transient failures make it non-trivial to ensure that all non-faultymachines agree upon the new authority location. The ambiguity thatarises due to transient failures can be achieved automatically by aconsensus protocol such as Paxos, hot-warm failover schemes, via manualintervention by a remote system administrator, or by a local hardwareadministrator (such as by physically removing the failed machine fromthe cluster, or pressing a button on the failed machine). In someembodiments, a consensus protocol is used, and failover is automatic. Iftoo many failures or replication events occur in too short a timeperiod, the system goes into a self-preservation mode and haltsreplication and data movement activities until an administratorintervenes in accordance with some embodiments.

As authorities are transferred between storage nodes and authorityowners update entities in their authorities, the system transfersmessages between the storage nodes and non-volatile solid state storageunits. With regard to persistent messages, messages that have differentpurposes are of different types. Depending on the type of the message,the system maintains different ordering and durability guarantees. Asthe persistent messages are being processed, the messages aretemporarily stored in multiple durable and non-durable storage hardwaretechnologies. In some embodiments, messages are stored in RAM, NVRAM andon NAND flash devices, and a variety of protocols are used in order tomake efficient use of each storage medium. Latency-sensitive clientrequests may be persisted in replicated NVRAM, and then later NAND,while background rebalancing operations are persisted directly to NAND.

Persistent messages are persistently stored prior to being replicated.This allows the system to continue to serve client requests despitefailures and component replacement. Although many hardware componentscontain unique identifiers that are visible to system administrators,manufacturer, hardware supply chain and ongoing monitoring qualitycontrol infrastructure, applications running on top of theinfrastructure address virtualize addresses. These virtualized addressesdo not change over the lifetime of the storage system, regardless ofcomponent failures and replacements. This allows each component of thestorage system to be replaced over time without reconfiguration ordisruptions of client request processing.

In some embodiments, the virtualized addresses are stored withsufficient redundancy. A continuous monitoring system correlateshardware and software status and the hardware identifiers. This allowsdetection and prediction of failures due to faulty components andmanufacturing details. The monitoring system also enables the proactivetransfer of authorities and entities away from impacted devices beforefailure occurs by removing the component from the critical path in someembodiments.

Referring to FIGS. 1-4 , in addition to component redundancy in thecommunication channel, storage cluster 160 is configured to allow forthe loss of one or more storage nodes 150. In some embodiments thiscluster redundancy level may be one for relatively small storageclusters 160 (less than 8 storage nodes 150) and two for relativelylarger storage clusters 160 (8 or more storage nodes 150) although anynumber would be suitable for the cluster redundancy level. In someembodiments, where more storage nodes 150 than the redundancy level arelost, the storage cluster 160 cannot guarantee availability of data orintegrity of future updates. As mentioned above, data redundancy isimplemented via segments. A segment is formed by selecting equal sizedshards from a subset of the non-volatile solid state storage 152, eachwithin a different storage node 150. Shards are reserved to establishthe redundancy level, e.g., one or two, and then a remainder constitutesthe data (the data shards). The shards are encoded using an ECC schemesuch as parity or Reed-Soloman (RAID 6), so that any subset of theshards equal in count to the data shards may be used to reconstruct thecomplete data. The storage cluster redundancy represents a minimum levelof redundancy and it may be exceeded for any individual data element.Segments are stored as a set of non-volatile solid state storage units,roles (data position or parity) and allocation unit local to eachnon-volatile solid state storage unit. The allocation units may be aphysical address or an indirection determined within the non-volatilesolid state storage 152. Each shard may be portioned into pages and eachpage into code words. In some embodiments, the pages are between about 4kilobytes (kB) and 64 kB, e.g., 16 kB, while the code words are betweenabout 512 bytes to 4 kB, e.g., 1 kB. These sizes are one example and notmeant to be limiting as any suitable size for the code words and thepages may be utilized. The code words contain local error correction anda checksum to verify the error correction was successful. This checksumis “salted” with the logical address of the contents meaning that afailure to match the checksum may occur if the data is uncorrectable ormisplaced. In some embodiments, when a code word fails a checksum it isconverted to an “erasure” for purpose of the error correction algorithmso that the code word may be rebuilt.

If storage nodes 150 are added to the storage cluster 160, the targetsegment width (data shards and parity shards) changes. Newly allocatedsegments can adopt these parameters immediately. If the new clusterconfiguration is too narrow to allow segments to be rebuilt to thetarget redundancy the segment is replaced with a new segment. If storagecluster 160 has increased the target redundancy an extra redundant shardcan be allocated and generated without changing the segment contents.All other segments may remain in place without modification, leaving thesystem with multiple concurrent segment dimensions. When the targetsegment is getting wider and more efficient this information can becombined with other information to determine if the segment is a bettercandidate than most for background processes like wear level or garbagecollection. Storage nodes 150 can have differing non-volatile solidstate storage 152 of differing sizes in some embodiments. If there aremany units of non-volatile solid state storage 152 of each size then thenormal allocation rules for segments may apply and the larger size unitsof non-volatile solid state storage 152 will have more overlappingsegments. The storage cluster 160 may also decide to ignore the excessspace or allocate segments of narrower width to make use of the extraspace.

Larger storage clusters 160 may be divided into storage node groups toincrease the redundancy without increasing segment width. As an example,a system of 28 storage nodes 150 may be divided into two groups of 14each with a segment size of 10+2. When a segment is allocated the systemis configured to not select shards from storage nodes in multiplegroups. This arrangement ensures that up to four storage nodes may belost, i.e., two from each group and the system operates normally.Storage node groups may be aligned with a chassis to take advantage of ahigher bandwidth communications interconnect 170 for redundantoperations. Storage node groups can be combined without any segmentreconfiguration. If a storage node group is partitioned, any segmentthat crosses the two storage node is partitioned. These partitionedsegments which are allocated in two storage node groups have theirshards realigned within storage nodes before the storage node groups areconsidered independent. The system may wait for shards to be rewrittenaccording to a normal schedule, rebuild shards into the same storagenode group or move the contents into another segment.

The total set of non-volatile solid state storage units that have atleast one shard allocated to a segment may be referred to as thereferenced set. These non-volatile solid state storage units (via theirstorage nodes) are organized first into storage node groups, with aredundancy level within each storage node group. Any subset thatfulfills the redundancy level in every storage node group is a quorumset, where the entire storage cluster may operate normally. The currentset of non-volatile solid state storage units that are operating and maybe accessed directly or indirectly via the communication buses isreferred to as the reachable set. The storage cluster may be consideredto be operating when the reachable set forms a valid quorum. Devicesthat are referenced but not reachable may be referred to as phantomdevices. All data that is stored on the device may be reconstructed fromother shards in the same segment. The storage cluster attempts todereference any phantom device by persistently storing the rebuiltshards on a reachable device. Any non-volatile solid state storage unitthat is not already within a storage node that contains a segment shardis a candidate for rebuilding. In some embodiments, rebuilding proceedsby ordinary data maintenance that would lead to deallocation of theoverlapping segment, similar to garbage collection. Heuristics thatdetermine the relative efficiency in terms of reading, writing orcomputation may be used to decide which path is appropriate. When nosegments map shards onto a particular non-volatile solid state storageunit the non-volatile solid state storage unit is considered evicted andis no longer required to participate in quorum. When there are no longerany phantom non-volatile solid state storage units, the storage clusteris at full redundancy and may lose further storage nodes while remainingin quorum.

In NVRAM, redundancy is not organized by segments but instead bymessages, where each message (128 bytes to 128 kB) establishes its owndata stripe. NVRAM is maintained at the same redundancy as segmentstorage and operates within the same storage node groups in someembodiments. Because messages are stored individually the stripe widthis determined both by message size and the storage clusterconfiguration. Larger messages may be more efficiently stored as widerstrips.

FIG. 5 is a multiple level block diagram, showing contents of a storagenode 150 and contents of a non-volatile solid state storage 152 of thestorage node 150. Data is communicated to and from the storage node 150by a network interface controller (NIC) 202 in some embodiments. Eachstorage node 150 has a CPU 156, and one or more non-volatile solid statestorage 152, as discussed above. Moving down one level in FIG. 5 , eachnon-volatile solid state storage 152 has a relatively fast non-volatilesolid state memory, such as nonvolatile random access memory (NVRAM)204, and flash memory 206. In some embodiments, NVRAM 204 may be acomponent that does not require program/erase cycles (DRAM, MRAM, PCM),and can be a memory that can support being written vastly more oftenthan the memory is read from. Moving down another level in FIG. 5 , theNVRAM 204 is implemented in one embodiment as high speed volatilememory, such as dynamic random access memory (DRAM) 216, backed up byenergy reserve 218. Energy reserve 218 provides sufficient electricalpower to keep the DRAM 216 powered long enough for contents to betransferred to the flash memory 206 in the event of power failure. Insome embodiments, energy reserve 218 is a capacitor, super-capacitor,battery, or other device, that supplies a suitable supply of energysufficient to enable the transfer of the contents of DRAM 216 to astable storage medium in the case of power loss. The flash memory 206 isimplemented as multiple flash dies 222, which may be referred to aspackages of flash dies 222 or an array of flash dies 222. It should beappreciated that the flash dies 222 could be packaged in any number ofways, with a single die per package, multiple dies per package (i.e.multichip packages), in hybrid packages, as bare dies on a printedcircuit board or other substrate, as encapsulated dies, etc. In theembodiment shown, the non-volatile solid state storage 152 has acontroller 212 or other processor, and an input output (I/O) port 210coupled to the controller 212. I/O port 210 is coupled to the CPU 156and/or the network interface controller 202 of the flash storage node150. Flash input output (I/O) port 220 is coupled to the flash dies 222,and a direct memory access unit (DMA) 214 is coupled to the controller212, the DRAM 216 and the flash dies 222. In the embodiment shown, theI/O port 210, controller 212, DMA unit 214 and flash I/O port 220 areimplemented on a programmable logic device (PLD) 208, e.g., a fieldprogrammable gate array (FPGA). In this embodiment, each flash die 222has pages, organized as sixteen kB (kilobyte) pages 224, and a register226 through which data can be written to or read from the flash die 222.In further embodiments, other types of solid-state memory are used inplace of, or in addition to flash memory illustrated within flash die222.

FIG. 6 is a flow diagram for a method of rebuilding data in a storagecluster. The method can be practiced on or by various embodiments of astorage cluster and storage nodes as described herein. Various steps ofthe method can be performed by a processor, such as a processor in astorage cluster or a processor in a storage node. Portions or all of themethod can be implemented in software, hardware, firmware orcombinations thereof. The method initiates with action 602, where userdata is distributed with erasure coding. For example, the user datacould be distributed across the storage nodes of a storage cluster usingone or more erasure coding schemes. Two erasure coding schemes (or more,in some embodiments) can coexist in the storage nodes in the storagecluster. In some embodiments, each storage node can determine which of aplurality of erasure coding schemes to apply when writing data, and candetermine which erasure coding scheme to apply when reading data. Theerasure coding schemes can be the same or differing erasure codingschemes.

The method proceeds to action 604 where the storage nodes in the storagecluster are checked. In some embodiments the storage nodes are checkedfor a heartbeat where each storage node periodically issues a messagewhich acts as the heartbeat. In alternative embodiments the checking isin the form of querying a storage node, and the lack of response to thequery indicates that the storage node is failing. In a decision action606, it is determined if a storage node is unreachable. For example, ifany storage nodes are no longer issuing heartbeats, or any storage nodesfail to respond to queries, or some combination of these or otherindications, one of the other storage nodes could determine that thestorage node is unreachable. If this is not the situation, flow branchesback to the action 602, in order to continue distributing user data,e.g., writing user data into the storage nodes as the user data arrivesfor storage. If it is determined that any storage node is unreachable,flow continues to action 608.

In the decision action 608, it is determined whether the data is to berebuilt. In some embodiments, the data is rebuilt after one storage nodebecomes unreachable, and in some embodiments the data is rebuilt aftertwo storage nodes become unreachable. In variations, other factors couldinfluence the decision as to whether or not to rebuild data. Forexample, a critical task could be in progress, and the decision torebuild the data could be delayed. Various mechanisms that could betaken into account in the decision to rebuild the data include errorcorrection counts, error correction rates, failures of reads, failuresof writes, loss of heartbeat, failure to reply to queries, and so on.Appropriate modification to the method of FIG. 6 is readily understoodfor these and further embodiments. If the data is not to be rebuilt,flow branches back to the action 602, in order to continue distributinguser data. If the data is to be rebuilt, flow proceeds to action 610.Entities owned by the unreachable storage node are reassigned to anotherone of the storage nodes, in action 610. For example, the entities couldbe data units or metadata as discussed above and the entities could begoverned by an authority of a storage node. This reassignment ofentities could result in the storage nodes determining that a particularerasure coding scheme is suitable for rebuilding user data, as occurs inactions 618 and 620. The method advances to action 612 where the data isrecovered using erasure coding. The storage nodes could apply theappropriate erasure coding scheme to recover the user data.

In the action 612, the data is recovered using erasure coding. Therecovery could be achieved according to examples of erasure coding asdiscussed above. More specifically, the data is recovered from theremaining storage nodes, e.g., using error correcting code or readingfrom a remaining storage node, as appropriate. Data could be recoveredusing two or more types of erasure coding, in cases where these two ormore types of erasure coding coexist in the storage nodes. In a decisionaction 614, it is determine if the data should be read in parallel. Insome embodiments, there is more than one data path, e.g., as with doubleredundancy of data, and the data could be read in parallel across twopaths. If the data is not to be read in parallel, flow branches to theaction 618. If the data is to be read in parallel, flow branches to theaction 616, in which the results are raced. The winner of the race isthen used as the recovered data.

In an action 618, the erasure coding scheme for rebuilding isdetermined. For example, in some embodiments each storage node candecide which of two or more erasure coding schemes to apply when writingthe data across the storage units. In some embodiments, the storagenodes cooperate to determine the erasure coding scheme. This cooperationmay be achieved by determining which storage node has responsibility forthe erasure coding scheme for a specific data segment, or by assigning astorage node to have this responsibility. In some embodiments, variousmechanisms such as witnessing, voting, or decision logic, and so on, areemployed to achieve this action. Non-volatile solid state storage unitsmay act as witnesses or voters in some embodiments. Thus, if one copy ofan authority becomes defective, the remaining functioning non-volatilesolid state storage units and the remaining copies of the authoritiescan determine the contents of the defective authority. In an action 620,the recovered data is written across the remainder of the storage nodes,with erasure coding. For example, the erasure coding scheme that isdetermined for the rebuilding could be different from the erasure codingscheme that is applied in recovering the data, i.e., in reading thedata. In some embodiments, loss of two of the storage nodes may meanthat some erasure coding schemes are no longer applicable to theremaining storage nodes, and the storage nodes then switch to an erasurecoding scheme that is applicable to the remaining storage nodes.

It should be appreciated that the methods described herein may beperformed with a digital processing system, such as a conventional,general-purpose computer system. Special purpose computers, which aredesigned or programmed to perform only one function may be used in thealternative. FIG. 7 is an illustration showing an exemplary computingdevice which may implement the embodiments described herein. Thecomputing device of FIG. 7 may be used to perform embodiments of thefunctionality for a storage node or a non-volatile solid state storagein accordance with some embodiments. The computing device includes acentral processing unit (CPU) 701, which is coupled through a bus 705 toa memory 703, and mass storage device 707. Mass storage device 707represents a persistent data storage device such as a disc drive, whichmay be local or remote in some embodiments. The mass storage device 707could implement a backup storage, in some embodiments. Memory 703 mayinclude read only memory, random access memory, etc. Applicationsresident on the computing device may be stored on or accessed via acomputer readable medium such as memory 703 or mass storage device 707in some embodiments. Applications may also be in the form of modulatedelectronic signals modulated accessed via a network modem or othernetwork interface of the computing device. It should be appreciated thatCPU 701 may be embodied in a general-purpose processor, a specialpurpose processor, or a specially programmed logic device in someembodiments.

Display 711 is in communication with CPU 701, memory 703, and massstorage device 707, through bus 705. Display 711 is configured todisplay any visualization tools or reports associated with the systemdescribed herein. Input/output device 709 is coupled to bus 505 in orderto communicate information in command selections to CPU 701. It shouldbe appreciated that data to and from external devices may becommunicated through the input/output device 709. CPU 701 can be definedto execute the functionality described herein to enable thefunctionality described with reference to FIGS. 1-6 . The code embodyingthis functionality may be stored within memory 703 or mass storagedevice 707 for execution by a processor such as CPU 701 in someembodiments. The operating system on the computing device may beMS-WINDOWS™, UNIX™, LINUX™, iOS™, CentOS™, Android™, Redhat Linux™,z/OS™, or other known operating systems. It should be appreciated thatthe embodiments described herein may be integrated with virtualizedcomputing system also.

Detailed illustrative embodiments are disclosed herein. However,specific functional details disclosed herein are merely representativefor purposes of describing embodiments. Embodiments may, however, beembodied in many alternate forms and should not be construed as limitedto only the embodiments set forth herein.

It should be understood that although the terms first, second, etc. maybe used herein to describe various steps or calculations, these steps orcalculations should not be limited by these terms. These terms are onlyused to distinguish one step or calculation from another. For example, afirst calculation could be termed a second calculation, and, similarly,a second step could be termed a first step, without departing from thescope of this disclosure. As used herein, the term “and/or” and the “/”symbol includes any and all combinations of one or more of theassociated listed items.

As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”,“comprising”, “includes”, and/or “including”, when used herein, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. Therefore, the terminology usedherein is for the purpose of describing particular embodiments only andis not intended to be limiting.

It should also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the figures.For example, two figures shown in succession may in fact be executedsubstantially concurrently or may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

With the above embodiments in mind, it should be understood that theembodiments might employ various computer-implemented operationsinvolving data stored in computer systems. These operations are thoserequiring physical manipulation of physical quantities. Usually, thoughnot necessarily, these quantities take the form of electrical ormagnetic signals capable of being stored, transferred, combined,compared, and otherwise manipulated. Further, the manipulationsperformed are often referred to in terms, such as producing,identifying, determining, or comparing. Any of the operations describedherein that form part of the embodiments are useful machine operations.The embodiments also relate to a device or an apparatus for performingthese operations. The apparatus can be specially constructed for therequired purpose, or the apparatus can be a general-purpose computerselectively activated or configured by a computer program stored in thecomputer. In particular, various general-purpose machines can be usedwith computer programs written in accordance with the teachings herein,or it may be more convenient to construct a more specialized apparatusto perform the required operations.

A module, an application, a layer, an agent or other method-operableentity could be implemented as hardware, firmware, or a processorexecuting software, or combinations thereof. It should be appreciatedthat, where a software-based embodiment is disclosed herein, thesoftware can be embodied in a physical machine such as a controller. Forexample, a controller could include a first module and a second module.A controller could be configured to perform various actions, e.g., of amethod, an application, a layer or an agent.

The embodiments can also be embodied as computer readable code on anon-transitory computer readable medium. The computer readable medium isany data storage device that can store data, which can be thereafterread by a computer system. Examples of the computer readable mediuminclude hard drives, network attached storage (NAS), read-only memory,random-access memory, CD-ROMs, CD-Rs, CD-RWs, magnetic tapes, and otheroptical and non-optical data storage devices. The computer readablemedium can also be distributed over a network coupled computer system sothat the computer readable code is stored and executed in a distributedfashion. Embodiments described herein may be practiced with variouscomputer system configurations including hand-held devices, tablets,microprocessor systems, microprocessor-based or programmable consumerelectronics, minicomputers, mainframe computers and the like. Theembodiments can also be practiced in distributed computing environmentswhere tasks are performed by remote processing devices that are linkedthrough a wire-based or wireless network.

Although the method operations were described in a specific order, itshould be understood that other operations may be performed in betweendescribed operations, described operations may be adjusted so that theyoccur at slightly different times or the described operations may bedistributed in a system which allows the occurrence of the processingoperations at various intervals associated with the processing.

In various embodiments, one or more portions of the methods andmechanisms described herein may form part of a cloud-computingenvironment. In such embodiments, resources may be provided over theInternet as services according to one or more various models. Suchmodels may include Infrastructure as a Service (IaaS), Platform as aService (PaaS), and Software as a Service (SaaS). In IaaS, computerinfrastructure is delivered as a service. In such a case, the computingequipment is generally owned and operated by the service provider. Inthe PaaS model, software tools and underlying equipment used bydevelopers to develop software solutions may be provided as a serviceand hosted by the service provider. SaaS typically includes a serviceprovider licensing software as a service on demand. The service providermay host the software, or may deploy the software to a customer for agiven period of time. Numerous combinations of the above models arepossible and are contemplated.

Various units, circuits, or other components may be described or claimedas “configured to” perform a task or tasks. In such contexts, the phrase“configured to” is used to connote structure by indicating that theunits/circuits/components include structure (e.g., circuitry) thatperforms the task or tasks during operation. As such, theunit/circuit/component can be said to be configured to perform the taskeven when the specified unit/circuit/component is not currentlyoperational (e.g., is not on). The units/circuits/components used withthe “configured to” language include hardware—for example, circuits,memory storing program instructions executable to implement theoperation, etc. Reciting that a unit/circuit/component is “configuredto” perform one or more tasks is expressly intended not to invoke 35U.S.C. 112, sixth paragraph, for that unit/circuit/component.Additionally, “configured to” can include generic structure (e.g.,generic circuitry) that is manipulated by software and/or firmware(e.g., an FPGA or a general-purpose processor executing software) tooperate in manner that is capable of performing the task(s) at issue.“Configured to” may also include adapting a manufacturing process (e.g.,a semiconductor fabrication facility) to fabricate devices (e.g.,integrated circuits) that are adapted to implement or perform one ormore tasks.

The foregoing description, for the purpose of explanation, has beendescribed with reference to specific embodiments. However, theillustrative discussions above are not intended to be exhaustive or tolimit the invention to the precise forms disclosed. Many modificationsand variations are possible in view of the above teachings. Theembodiments were chosen and described in order to best explain theprinciples of the embodiments and its practical applications, to therebyenable others skilled in the art to best utilize the embodiments andvarious modifications as may be suited to the particular usecontemplated. Accordingly, the present embodiments are to be consideredas illustrative and not restrictive, and the invention is not to belimited to the details given herein, but may be modified within thescope and equivalents of the appended claims.

What is claimed is:
 1. A storage system, comprising: a plurality ofstorage nodes configured to communicate together; each of the pluralityof storage nodes having nonvolatile solid-state memory for data storage;the plurality of storage nodes configured to recover data writtenaccording to a first erasure coding scheme; and the plurality of storagenodes configured to write the recovered data according to a seconderasure coding scheme.
 2. The storage cluster of claim 1, wherein thesolid-state memory includes flash memory.
 3. The storage cluster ofclaim 1, wherein two differing erasure codes coexist in one of theplurality of storage nodes.
 4. The storage cluster of claim 1, whereindata recovery is initiated responsive to a lack of a response to aquery.
 5. The storage cluster of claim 1, further comprising: theplurality of storage nodes configured to initiate data recovery based onlack of a heartbeat of one of the plurality of storage nodes that isunreachable.
 6. The storage cluster of claim 1, wherein data recovery isinitiated independent of an attempt to read the data.
 7. The storagecluster of claim 1, wherein to recover the data comprises: reading aredundant copy of the data in accordance with the first erasure codingscheme.
 8. The storage cluster of claim 1, wherein recovery of the datawritten according to the first erasure coding scheme is responsive toone of the plurality of storage nodes being unreachable.
 9. The storagecluster of claim 1, wherein segments of the data are owned by differingauthorities.
 10. A method, comprising: reading data distributed across aplurality of storage nodes, using a first erasure coding schemeresponsive to a determination to rebuild the data for one of theplurality of storage nodes; and writing the read data across theplurality of storage nodes, using a second erasure coding scheme,wherein each storage node of the plurality of storage nodes isconfigured to decide whether the second erasure coding scheme is one ofa same erasure coding scheme or a differing erasure coding scheme fromthe first erasure coding scheme.
 11. The method of claim 10, whereinreading the data across the plurality of storage nodes using a firsterasure coding scheme comprises: recovering the data in accordance witha data striping scheme governed by the first erasure coding scheme. 12.The method of claim 10, further comprising: determining that the one ofthe plurality of storage nodes is unreachable based on a timeoutassociated with a read of the data.
 13. The method of claim 10, furthercomprising: determining that the one of the plurality of storage nodesis unreachable is independent of a request from outside the plurality ofstorage nodes to read the data.
 14. The method of claim 10, wherein theplurality of storage nodes are distributed across a plurality ofchassis.
 15. The method of claim 10, further comprising: reassigning, toone of the remainder of the plurality of storage nodes, entities ownedby the unreachable storage node, wherein the entities include units ofdata.
 16. The method of claim 10, wherein segments of the data are ownedby differing authorities.
 17. A tangible, non-transitory,computer-readable media having instructions thereupon which, whenexecuted by a processing device, cause the processing device to: readdata distributed across a plurality of storage nodes, using a firsterasure coding scheme responsive to a determination to rebuild the datafor one of the plurality of storage nodes; and write the read dataacross the plurality of storage nodes, using a second erasure codingscheme.
 18. The computer readable media of claim 17, wherein reading thedata across the plurality of storage nodes using a first erasure codingscheme comprises: recovering the data in accordance with a data stripingscheme governed by the first erasure coding scheme.
 19. The computerreadable media of claim 17, wherein the first erasure coding scheme isdifferent than the second erasure coding scheme.
 20. The computerreadable media of claim 17, wherein the processor is further configuredto: determine that the one of the plurality of storage nodes isunreachable is independent of a request from outside the plurality ofstorage nodes to read the data.